Device for detecting a space between adjacent blocks of data recorded in a recording medium

ABSTRACT

A detector for detecting a space between blocks of data recorded on a recording medium, comprising a recording medium with a given speed, which provides a reproduction signal corresponding to an information recorded on the recording medium and issues a transport operation indicating signal; a discrimination means for discriminating the transport operation indicating signal, which provides a filter characteristic changing signal in accordance with the transport operation indicating signal; and a variable filter means for filtering the reproduction signal and providing a timing signal, of which filtering characteristic is changed in accordance with the filter characteristic changing signal. An operation mode of the recording medium transport means is changed by a transport control signal corresponding to the timing signal.

This is a continutation, of application Ser. No. 361,789 now U.S. Pat. No. 4,442,464, filed Mar. 25, 1982, which is in turn a continuation of Ser. No. 151,916, filed 5-21-80, now abandoned.

FIELD OF THE INVENTION

This invention relates to improvements to a device for detecting a space between adjacent blocks of data recorded in a magnetic tape. This space detector is adapted to be used particularly with a device for automatically selecting a desired music piece from among those recorded in the magnetic tape.

BACKGROUND OF THE INVENTION

In recent years, combination radio-cassette tape recorder/players have begun rapidly enjoying wide acceptance. This radio-cassette tape record/player must have a large power output, have high fidelity (HiFi) and incorporate many different functions.

In the field of audio equipment, it is desired to develop a device which can automatically select a desired music piece from among those recorded in a magnetic tape and allow the user himself (or herself) to enjoy the music and, if desired, to sing along. Also tape recorder/players using a tape recording of an orchestral accompaniment devoid of a vocal performance are enjoying an unprecedented boom.

The radio-cassette tape record/players must be able to produce a vocal-free orchestral accompaniment relatively easily. A known cuing apparatus or an automatic music piece selector developed from said cuing apparatus enables the ordinary tape recorder/player to meet this requirement with relative ease. However, a tape recorder/player with such an apparatus must not simply cope with a demand for the production of a vocal-free orchestral accompaniment, but it should also be capable at the same time of carrying out any operation mode reliably without an erroneous behavior such as a failure to select a desired music piece.

The known automatic music piece selector (even a simple cuing apparatus is regarded as similar to such a selector) comprises a coincidence detector which counts detection pulses issued from an inter-music piece space detector (the inter-music piece space detector detects a signal-free space between the respective adjacent music pieces), compares the counted number of detection pulses with a value preset in a designator of the sequential number of a music piece (hereinafter referred to as a "music piece sequence designator"), and sends forth a coincidence signal when the compared numbers accord with each other or indicate a prescribed conforming relationship.

The above-mentioned automatic music piece selector automatically selects a desired music piece by actuating an electro-mechanical conversion mechanism, such as a plunger, by said coincidence signal and changing over the operation mode of a tape recorder or player. This automatic music piece-selecting operation is carried out by letting a tape transport proceed at high speed (fast forward or rewind mode) until a specified or desired music piece is detected, stopping the high speed tape transport after detection of a desired music piece, and finally changing the operation mode to the specified speed of the tape transport (playback mode).

The above-mentioned conventional automatic music piece selector automatically selects only one music piece by a single operation. An operation of detecting an inter-music piece space is undertaken while the tape is made to run at high speed. Therefore, said conventional music piece selector is only capable of carrying out the simplest function among those which are possible effected by such device.

Moreover, the prior art music piece selector had the serious drawback that a device for detecting a space between adjacent music pieces or data recorded in a tape (hereinafter referred to as a "space detector") sometimes failed to detect the space. The known space detector comprises a clipper which amplifies a playback signal up to a clipping level, a rectifier which rectifies an output signal from the clipper, a Schmitt trigger which distinguishes between the levels of rectified outputs from the rectifier, and a differentiating circuit which converts an output from the Schmitt trigger into a pulse signal. The above-mentioned prior art space detector is generally of the type which generates a pulse signal for each space between adjacent music pieces or data recorded in a tape by utilizing the different levels of signals denoting a recorded signal portion representing a music piece and a signal-free space between adjacent recorded music pieces or data. The arrangement of such a space detector is set forth in the Japanese Patent Disclosure No. 152,210 (1977) on an apparatus for detecting a space between adjacent music pieces recorded in a tape used with a tape recorder.

There are two reasons that the prior art space detector failed to reliably detect an inter-music piece space, possibly leading to an erroneous action. One of these two reasons is low frequency noises such as hums which generally arise from a signal-free space between adjacent music pieces recorded in a tape. Where a tape runs at a high speed for detection of an inter-music piece space, these low frequency noises are undesirably reproduced with a high level and at an appreciably high frequency of, for example, about 3 KHz. Such high frequency noises arising from the signal-free inter-music piece space are sometimes detected without being distinguished from music signals. Therefore, the space detector fails to reliably detect an inter-music piece space.

The other of the two reasons is related to the timing in which the changeover mechanism of a tape recorder is actuated for a desired operation mode. When a tape player is changed over from the playback mode to the fast forward mode (FF mode) for the purpose of selecting a music piece while another music piece is still being reproduced, coincidence of the space count and the present value does not always take place between the point of time at which the mechanical system begins to be operated and the point of time at which the electric system beings to be actuated.

The conventional space detector controls the operation of a tape recorder by an output denoting the detection of an inter-music piece space (strictly speaking, a signal showing a coincidence between a connected number of pulses issued from the space detector and a value preset in the music piece sequence designator). At this time, the operation of the space detector is controlled by a discrimination signal whose logic level varies with the operation mode of the tape player mechanism. For example, is set at "1" at the time of, for example, the playback mode, and at "0" at the time of the fast forward (FF) mode. If, therefore, no coincidence is ensured between the time when the mechanical system of a tape recorder begins to be operated and the time when the electric system thereof begins to be actuated, then the space detector undesirably takes an erroneous action.

Another problem is that where an electro-mechanical conversion mechanism, such as a plunger driven by a signal denoting an inter-music piece space, is actuated, then the so-called plunger noises arise, sometimes causing the space detector to take an erroneous action.

In recent years, an automatic, highly advanced, music piece selector has been required to pick up not merely only one music piece, but also a plurality of music pieces at once. This requirement further aggravates the aforesaid difficulties. Moreover, a new problem characteristic of the selection of plural music pieces is now brought about. Where a single music piece is to be selected, it is usually sufficient if an inter-music piece space is detected simply by the fast forward mode. Conversely, where plural music pieces are to be picked up, it is necessary to detect an inter-music piece space not only during the fast forward mode, but also during the playback mode. Therefore, it has become impossible to simply treat low frequency noises such as hums arising from an inter-music piece space.

SUMMARY OF THE INVENTION

It is accordingly the object of this invention to provide a device for exactly detecting one or more of plural blocks of data recorded in a recording medium such as a magnetic tape regardless of the speed at which the tape travels. Or more concretely, the object of the invention is to provide a space detector which can unfailingly detect a space between adjacent music pieces or data blocks recorded in the magnetic tape without being affected by low frequency noises arising from an inter-music piece space, regardless of whether the magnetic tape runs at the fast forward mode or playback mode.

To achieve the above-mentioned object, a detector according to the invention comprises a variable filter means whose frequency characteristic is of the lower frequency range decreasing type and whose cut-off frequency is varied dependent on the transport speed of the recording medium. More specifically, the filter means is a low-pass filter or the like, and its cut-off frequency f_(c) at the -3dB response point can be changed. For example, the frequency f_(c) is high at fast forward (FF) mode and low at playback mode so as to prevent an erroneous detecting of the spacing portion due to low frequency noises, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a basic configuration of a detector embodying the invention;

FIG. 2A shows a frequency characteristic of a variable filter means 16 shown in FIG. 1 in which a cut-off frequency f_(c) is set at high value (f_(c1));

FIG. 2B shows a frequency characteristic of the variable filter means 16 in which the cut-off frequency f_(c) is set at low value (f_(c2));

FIG. 3 shows a detail block diagram of FIG. 1;

FIG. 4 shows a detailed circuit configuration of the signal treatment means 18 shown in FIG. 3;

FIG. 5 shows a modification of the circuit configuration shown in FIG. 4;

FIG. 6A shows wave forms at the corresponding points of the circuit configuration shown in FIG. 4 or 5 wherein the masking circuit 18₅ is not provided;

FIG. 6B shows wave forms at the corresponding points of the configuration shown in FIG. 4 or 5 wherein the masking circuit 18₅ is provided;

FIGS. 7A and 7B are timing charts illustrating the operation of the masking circuit 18₅ shown in FIG. 3, 4 or 5;

FIG. 8 explains how a muting phenomenon occurs with a mode change of the tape transport mechanism 10₁ shown in FIG. 3;

FIG. 9 shows another embodiment of the invention;

FIGS. 10, 10A and 11 show a modification of the variable filter means 16, whose cut-off frequency f_(c) is varied according to the tape transporting speed;

FIGS. 12 to 18 show various modifications of the variable filter means 16; and

FIG. 19 shows a typical characteristic of FET used for a variable impedance element shown in FIG. 17 or 18, whose inner resistance R=∂V_(DS) /∂I_(D) is varied in accordance with the gate potential -V_(G).

DETAILED DESCRIPTION OF THE INVENTION

Now, preferred embodiments of the invention will be described. To avoid a redundant description, common or similar reference numerals are used for designating common or similar elements. That is, any components or any elements recited by like reference numerals throughout the drawings are exchangeable directly or with minor changes that would not be difficult for one skilled in the art.

FIG. 1 shows a basic configuration of a detector of this invention. A recording medium transport means 10 includes, for example, a conventional tape transport mechanism, a control mechanics or control electronics for controlling the operation mode of the transport mechanism and a signal treatment electronics. The mechanism transports a recording medium, e.g. a music cassette tape, at one of two predetermined constant speeds. In the standard compact cassette the constant speed of a playback/recording mode is 4.8 cm/s, and a higher speed corresponds to cuing, fast forward (FF) or rewind (REW) mode of the transport mechanism. When the music cassette tape is transported, the signal treatment electronics or playback equalizer (EQ) amplifier produces a playback signal or reproduction signal E10 in the any mode, playback, cue, FF or REW, unless a muting circuit is activated which is provided within signal treatment electronics of the tape recorder/player (not shown). The control mechanics or the control electronics of the transport means 10 issues a transport operation indicating signal E12 which indicates the operation mode of the transport mechanism. For example, when the operation mode is the playback mode, the signal E12 is logical "1", and when it is the FF mode the signal E12 is logical "0".

The transport operation indicating signal E12 is supplied to a discrimination means 12. The means 12 is generally formed of a mechanical or electrical switch interlocking with the control mechanics or the control electronics. It provides a filter characteristic changing signal E14 and a time constant changing signal E16 upon receipt of signal E12.

The reproduction signal E10 and the filter characteristic changing signal E14 are supplied to a variable filter means 16. The means 16 is formed of, for example, a saturation amplifier whose frequency characteristic is a high-pass filter type and whose cut-off frequency is variable. The cut-off frequency varies according to the signal E14. The means 16 is saturated with large amplitude of the signal E10 when the transport means 10 is set to the high speed tape transport mode, i.e. FF mode etc., but not always saturated when the transport means 10 is set to the playback mode. The means 16 provides a timing signal E18 which corresponds to the filtered-signal of E10.

The timing signal E18 is supplied to a signal treatment means 18. The means 18 includes a bistable multivibrator or Schmitt trigger circuit of variable time constant type. The signal E18 triggers the bistable multivibrator, and then the means 18 provides a transport control signal E20.

The actual time constant of the bistable multivibrator is varied by the time constant changing signal E16. For example, when the tape transport mechanism of the recording medium transport means 10 transports a music cassette tape at 4.8 cm/s, the logical level of the transport operation indicating signal E12 is "1". In this case, the cut-off frequency f_(c2) of the high-pass filter in the variable filter means 16 is relatively low as shown in FIG. 2B, and the time constant of the bistable multivibrator of the means 18 is relatively large. By contrast, when the tape transport speed is higher than 4.8 cm/s as in the FF mode, the logical level of the signal E12 becomes "0" and the cut-off frequency rises to f_(c1) as shown in FIG. 2A. Then, the time constant of the multivibrator is reduced to a value suitable for the operational condition of the multivibrator with the tape transport speed.

The transport control signal E20 is fed back to the control mechanics or control electronics of the recording medium transport means 10. Suppose the tape transport mechanism transports a music cassette tape in the FF mode. Then, the cut-off frequency f_(c1) is high as shown in FIG. 2A. Provided with such a low-cut type frequency characteristic, the variable filter means 16 does not respond to a noise component such as hum in low frequency range. Thus, only the musical component without low frequency noise is passed through the means 16 and the timing signal E18 relating to the music containing no noise is supplied to the signal treatment means 18. When a spacing portion or no musical signal recorded portion between two different music sources recorded on the music cassette passes by the magnetic head, the amplitude of the reproduction signal E10 decreases, and the amplitude of the signal E18 also decreases. When the amplitude of the signal E18 become lower than a given level, the bistable multivibrator of the means 18 is triggered to supply the transport control signal E20 to the transport means 10. In response to the signal E20 the means 10 stops the tape transport operation of FF mode, whereby the operation mode of the tape transport mechanism is changed to stop or playback mode.

In the FIG. 1 configuration there is also provided means for preventing an erroneous operation due to a mechanical delay response in the recording medium transport means 10. That is, the means 10 has a device to provide an erroneous operation preventing signal E22 when the tape transport mechanism is activated to change the operation mode of the mechanism. The signal E22 is supplied to the signal treatment means 18 immediately after the mechanism is activated. The transport control signal E20 is not produced when the signal E22 is provided. Accordingly, in any situation, the signal E20 is not derived from the means 18 so long as the signal E22 lasts.

The erroneous operation preventing means is useful, for example, in the following case. Assume now that the tape transport mechanism transports a tape in playback mode and then in the FF mode. The transport operation indication signal E12 is then changed from logical "1" to logical "0". This logical level change elevates the cut-off frequency of the variable filter means (high-pass filter) 16 and decreases the time constant of the means 18. Further, the mechanical actuation of the tape transport mechanism occurs after the logical level change of the signal E12. Without erroneous operation preventing means, the level of the timing signal E18 distributed over the audio frequency range is instantly lowered by the level change of the signal E12, regardless of whether the present playback position is not the spacing portion, and then the level down of the signal E18 will cause the means 18 to erroneously provide the signal E20.

FIG. 3 is a detail block diagram of FIG. 1 configuration. The recording medium transport means 10 includes a tape transport mechanism 10₁ provided with a playback head 10_(1a), a control electronics or transport control logic 10₂ for controlling the mechanism 10₁ through a instruction signal IS and an playback equalizer 10₃ for equalizing and amplifying an output of the head 10_(1a) to provide the reproduction signal or playback signal E10.

The playback signal E10 is applied to the non-inverted input of an operational amplifier 16₁ included in the variable filter means 16. The output of the amplifier 16₁ is connected through a resistor R16₁ to the inverted input thereof. The inverted input of the amplifier 16₁ is grounded via a series circuit of a resistor R16₂ and a capacitor C16₁. The junction point of the resistor R16₂ and the capacitor C16₁ is connected to the collector of an NPN transistor 16₂ via a capacitor C16₂. The emitter of the transistor 16₂ is grounded and the base thereof is applied with the filter characteristic changing signal E14 through a resistor R16₃. The non-inverted input of the amplifier 16₁ is grounded through a resistor R16₄. The transistor 16₂ acts as a switch. It is turned-on by logical "1" of the signal E14 and turned-off by logical "0" of the signal E14.

Suppose the amplifier 16₁ is a so-called "ideal operational amplifier" with its gain being regarded as infinite. Then, a transfer function G16 between the signal E10 to the signal E18 will be as follows:

(a) The signal E14 is logical "0". This corresponds, for example, to the FF mode of the tape transport mechanism 10₁. The transistor 16₂ is in OFF state. In this condition, a transfer function G16₁ is expressed as: ##EQU1## where S=j2πf (Hz), T1=C16₁ R16₁ and T2=C16₁ R16₂. The property of the function G16₁ corresponds to FIG. 2A. The maximum gain of the function G16₁ is obtained from the equation (1) as follows: ##EQU2## The cut-off frequency f_(c1) at the level of -3dB point from the maximum gain is also obtained from equation (1). For simplicity's sake, we assume that in equation (1): ##EQU3##

Then, equation (1) is simplified as below. ##EQU4##

From equation (1A) the cut-off frequency f_(c1) is represented by: ##EQU5## In a model PC-X40 manufactured by Toshiba Co., in which the detector of this invention is applied, the frequency f_(c1) is approximately 16 KHz in order to sufficiently decrease the lower frequency gain of the amplifier 16₁ so that a hum component in the playback signal E10, which is produced in the FF mode, will not have its frequency and level elevated. The detector is therefore prevented from operating erroneously.

When the tape transport speed in the FF mode is about 30 to 40 times higher than the playback speed (4.8 cm/s), for example, the frequency of hum rises from 60 Hz to 1.8 KHz to 2.4 KHz and playback level of the hum component is greatly elevated. However, when the cut-off frequency f_(c1) is set at more than 10 KHz, the level of the hum is suppressed by the high-pass filter operation of the variable filter means 16, thereby the level of the timing signal E18 from the amplifier 16₁ is too low to affect the transport control signal E20.

(b) The signal E14 is logical "1". This corresponds to the playback mode of the mechanism 10₁, and the transistor 16₂ is in ON state. In this condition, a transfer function G16₂ is expressed as: ##EQU6## where S=j2πf (Hz), T3=(C16₁ +C16₂)R16₁ and T4=(C16₁ +C16₂)R16₂. The property of the function G16₂ corresponds to FIG. 2B. The maximum gain of the function G16₁ is obtained from equation (5) as follows: ##EQU7## Here, we assume for simplicity that in equation (5): ##EQU8## Then equation (5) is simplified as follows: ##EQU9## From equation (5a) the cut-off frequency f_(c2) is represented as follows: ##EQU10## In said model PC-X40, the frequency f_(c2) is selected to be about 330 Hz so that the flat-frequency response of the amplifier 16₁ covers the main audio frequency range. Since the energy level of a voice and/or music source at the playback mode is between about 100 Hz to 4 KHz in most cases and the flat gain (maximum gain) of the amplifier 16₁ is sufficiently large, an f_(c2) ≃330 Hz can provide the timing signal E18 indicating the music portion and the spacing portion. For example, where G16₂ max ≃50 dB, f_(c2) ≃330 Hz and filtering characteristic is -6 dB/oct, the gain G16₂ at 60 Hz is about 35 dB. The gain G16₂ ≃35 dB at 60 Hz is large enough to indicate the existence of bass-instrument music, such as bass drums, in the case of said model PC-X40.

Generally, when the maximum gain G16_(max) is almost constant, the following relation is preferable to prevent the erroneous operation: ##EQU11## where f_(c1) is the cut-off frequency at the high speed tape transport, f_(c2) is the cut-off frequency at the playback or predetermined constant speed tape transport, V1 is the tape transport speed at the high speed tape transport (e.g. at the FF mode), and V2 is the playback speed (e.g. 4.8 cm/s). In the case of said model PC-X40, f_(c1) ≃16 KHz, f_(c2) ≃330 Hz and V1/V2≃30 to 40. Thus, in this model, the frequency ratio N=16×10³ /330≃48, and relation (9) is satisfied.

The ratio N should be determined according to the closed loop gain or the transfer function G16 of the variable filter means 16, the tape transport speeds V1, V2 and so on. However, there is a suitable range of the raio N, especially in a compact cassette tape recorder/player. If the speed ratio V1/V2 is varied from 10 to 100, the frequency ratio N is set as follows:

    N≧100

By contrast, if the ratio V1/V2 is 30 to 40 as in a compact cassette, the ratio N is selected to be:

    N≧40

The timing signal E18 derived from the amplifier 16₁ is wave-shaped by a wave shaper 18₁. The wave shaper 18₁ produces a trigger pulse E18₁ having sharp rising edge, which is supplied to a variable time constant Schmitt trigger or a variable time constant bistable multivibrator 18₂. The switching operation of the multivibrator 18₂ includes a suitable hysteresis to ensure a sufficient noise margin which prevents a mistriggering. When triggered by the pulse E18₁, the multivibrator 18₂ produces a first signal E18₂ the pulse width of which depends on the time constant of the multivibrator 18₂. The time constant of the multivibrator 18₂ is varied by the time constant changing signal E16. This change of the time constant is carried out simultaneously with said cut-off frequency change in the variable filter means 16 so that the pulse width of the signal E18₂ is well fitted to the speed at which the tape is transported by the tape transport mechanism 10₁.

The signal E18₂ is wave-shaped by a wave shaper 18₃. The shaper 18₃ provides a pulse signal E18₃ having narrow pulse width arising at the level change portion of the signal E18₂. The signal E18₃ is applied to the first input of an AND gate 18₄, while the second input thereof is supplied with a second signal E18₄ derived from a masking circuit 18₅. The circuit 18₅ may be formed of a monostable multivibrator triggered by the erroneous operation preventing signal E22.

The signal E22 is derived from the transport control logic 10₂ when the logic 10₂, by the instruction signal IS, instructs the mechanism 10₁ to change its operation mode, e.g., changing FF mode to playback mode or vice versa. The logical AND of the signal E18₃ and E18₄ is supplied as the transport control signal E20 from the output of the AND gate 18₄ to the logic 10₂.

The components 18₁ to 18₅ constitute the signal treatment means 18.

The discrimination means 12 in FIG. 3 is formed of a leaf switch 12₁ and a registor R12. One end of the resistor R12 is applied with a positive potential V_(c), and the other end is grounded via the switch 12₁. The signals E14 and E16 are obtained from the junction point of the resistor R12 and the switch 12₁. The switch 12₁ is switched, interlocking with the mechanical operation of the tape transport mechanism 10₁. For example, the switch 12₁ is turned-on when the mechanism 10₁ is set to FF mode and turned-off when the mechanism 10₁ is set to playback mode. The ON state of the switch 12₁ makes the signals E14 and E16 have logical value a "0", and the OFF state makes them have a logical value "1". Thus, the mechanical ON/OFF instruction for the switch 12₁ is the same as the transport operation indicating signal E12.

FIG. 4 is a detail circuit configuration of the signal treatment means 18 shown in FIG. 3. The timing signal E18 is supplied to the base of a PNP transistor Q18₁ via a capacitor C18₁. The base and emitter of the transistor Q18₁ are connected to the potential +V_(c) respectively through resistors R18₁ and R18₂. The trigger signal E18₁ is derived from the collector of the transistor Q18₁. The components C18₁, Q18₁, R18₁ and R18₂ constitute the wave shaper 18₁.

The signal E18₁ is supplied to the inverted input of an amplifier A18. The inverted input is grounded via a parallel circuit of a capacitor C18₂ and a resistor R18₃. The inverted input is also grounded via a series circuit of a resistor R18₄ and the emitter-collector path of a PNP transistor Q18₂. The base of the transistor Q18₂ is supplied with the time constant changing signal E16 through a resistor R18₅. The output of the amplifier A18 is connected through a resistor R18₆ to the non-inverted input thereof, and the non-inverted input is grounded via a series circuit of resistors R18₇ and R18₈. The junction point of the resistors R18₇ and R18₈ is connected through a resistor R18₉ to the potential +V_(c).

The components A18, Q18₂, C18₂ and R18₃ to R18₉ constitute the variable time constant bistable multivibrator 18₂. The hysteresis property of the multivibrator 18₂ depends on the resistors R18₆ to R18₉ included in a positive feedback loop of the amplifier A18. The time constant depends on the capacitor C18₂ and the resistors R18₃, R18₄. That is, when the signal E16 is logical "1" at playback mode, the transistor Q18₂ is in OFF state. Therefore, the time constant is C18₂ R18₃. (If the input resistance of the amplifier A18 is as low as that of the resistor R18₃, the resistor R18₃ may be omitted. In this case the input resistance at the inverted input of the amplifier A18 is used for R18₃.)

On the other hand, when the signal E16 is logical "0" at FF mode, the transistor Q18₂ is in ON state, and the time constant is changed from C18₂ R18₃ to C18₂ (R18₃ //R18₄). More precisely, if C18₂ =3.3 μF, R18₃ =100K Ω and R18₄ =2.7K Ω, for example, the time constant at the playback mode=330 ms (C18₂ R18₃), whereas the time constant at the FF mode is C18₂ (R18₃ //R18₄)≃8.7 ms.

Now it will be described why such time constant change is required.

In the case of playback mode, the signal E18₁ usually corresponds to a music source. If the music is a classical one, there sometimes appears a very low level part (pp˜ppp) or no sound portion which lasts for a few seconds. Such low level or no sound portion should not be mistaken for a spacing portion between the adjacent music sources. To prevent mistriggering the multivibrator 18₂ due to such low or no sound portion of a few seconds, the time constant (C18₂ R18₃ is selected so large. In the case of FF (or REW) mode where the tape transport speed is so fast, as much as 30 to 40 times the playback speed, the time interval of the spacing portion will be approximately 0.1 to 0.2 second when the time interval of the spacing portion at the playback speed is 4 to 6 seconds. In this case, if the time constant of the multivibrator 18₂ is a large as C18₂ R18₃ =330 ms, the multivibrator 18₂ cannot detect a spacing portion and cannot be triggered even though the spacing portion appears. The time constant at the FF mode must therefore be lowered to, e.g. C18₂ (R18₃ //R18₄)≃8.7 ms.

The first signal E18₂ derived from the output of the amplifier A18 is applied to one end of a resistor R18₁₀. The other end of the resistor R18₁₀ is grounded via a capacitor C18₃ and a resistor R18₁₁. The junction point of the capacitor C18₃ and the resistor R18₁₁ is grounded through the cathode-anode path of a diode D18₁. The pulse signal E18₃ is obtained from the junction point of C18₃ and R18₁₁.

The components R18₁₀, R18₁₁, C18₃ and D18₁ constitute the wave shaper 18₃. The diode D18₁₀ is provided for absorbing the negative surge of the signal E18₃. The signal E18₃ is applied to the first input of the AND gate 18₄. The risk of destroying the input circuit element of the gate 18₄ is removed by the absorbing operation of the diode D18₂.

The erroneous operation preventing signal E22 is applied to the first input of an NAND gate G18₁. The output of the NAND gate G18₁ is connected through a capacitor C18₄ to the first input of a NAND gate 18₂. The first input of the gate G18₂ is grounded via a resistor R18₁₂ and also via the cathode-anode path of a diode D18₂. The output of the gate G18₂ is fed back to the second input of the gate G18₁, and the second input of the gate G18₂ is applied with the potential +V_(c) corresponding to logical value "1". The second signal E18₄ derived from the ouput of the NAND gate G18₂ is applied to the second input of the AND gate 18₄.

The components G18₁, G18₂, C18₄, R18₁₂ and D18₂ constitute the masking circuit or monostable multivibrator 18₅. The AND gate 18₄ provides the transport control signal E20.

With reference to FIG. 3 it will be briefly described how the FIG. 4 circuit operates. Suppose the tape transport mechanism 10₁ is set to playback mode and that the operation playback mode is changed to FF mode. Since the mode change by the mechanism 10₁ takes place inevitably with a time lag after the logical level changes of the signals E14 and E16 there is the risk that the multivibrator 18₂ is erroneously triggered at the time of the mode change of the playback mode to the FF mode. The moment said mode change occurs, the cut-off frequency f_(c) of the variable filter means 16 rises to, for example, 16 KHz and the time constant of the multivibrator 18₂ is reduced to, for example, 8.7 ms, despite the fact that mechanism 10₁ is still in playback mode. Evidently, the level of the timing signal E18, which depends on the playback signal E10, is instantaneously lowered and the multivibrator 18₂ is changed to a state in which the multivibrator 18₂ is easily triggered. Accordingly, even if the signal E10 include a music source (especially music played by a bass-instrument), the signal level at the inverted input of the amplifier A18 may become lower than that at the non-inverted input thereof, and the multivibrator 18₂ is mistriggered. There exists the possibility that the transport control signal E20 can be erroneously provided. Such an error may also be induced from a plunger noise, etc. The possibility of error can be completely removed by using the multivibrator (masking circuit) 18₅ and the AND gate 18₄. The operation of the multivibrator 18₅ and the AND gate 18₄ is as follows.

Now we assume that a time required for the complete mechanical mode change operation in the mechanism 10₁ is 600 ms at most, that the pulse width of the signal E18₃ is 40 ms and that the signals E18₃ and E18₄ appear almost at the same time. Where the time constant of the multivibrator 18₅ is so selected that the pulse width of the signal E18₄ is, for example, 750 ms, the margin of 150 ms (=750 ms-600 ms) is considered to prevent an unexpected erroneous operation.

On this assumption we operate the control logic 10₂ so as to change the playback mode to the FF mode in the mechanism 10₁. The multivibrator 18₅ is instantly triggered by the erroneous operation preventing signal E22. Immediately thereafter the multivibrator 18₅ produces the signal E18₄ having logical value "0" and a pulse width of 750 ms. As long as the signal E18₄ has a logical level of "0", the AND gate 18₄ is closed. Therefore, the gate 18₄ stops the signal E18₃ if produced by said mistriggering. The AND gate 18₄ passes the signal E18₃ after the logical level of the signal E18₄ returns to "1".

FIG. 5 is a modification of FIG. 4 configuration. In the modification the AND gate 18₄ is used as a buffer. The gating for the transport control signal E20 is performed by a PNP transistor Q18₃. The emitter of the transistor Q18₃ is connected to the output of the gate or buffer 18₄ and the collector thereof is grounded. To the base of the transistor Q18₃ the second signal E18₄ is applied through a resistor R18₁₃. As long as the signal E18₄ is logical "0", the output of the buffer 18₄ is shunted to ground via the emitter-collector path of the transistor Q18₃ which is in ON state. Accordingly the function of the FIG. 5 configuration is substantially the same as that of FIG. 4.

FIGS. 6A and 6B show wave forms at the corresponding points of the circurit configuration of FIG. 4 or 5. FIG. 6A shows wave forms obtained when the multivibrator 18₅ is not provided, and FIG. 6B is wave forms obtained when the multivibrator 18₅ is provided. In these figures, the wave form IQ18₁, corresponds to the collector current of the transistor Q18₁ of the wave shaper 18₁.

In FIG. 6A, pulses E18_(3a) and E18_(3b) are erroneous signals as described before with reference to FIGS. 4 and 5. That is, if the multivibrator 18₅ is used as mere buffer, said mistriggering occurs at the variable time constant Schmitt trigger 18₂. If this happens, the pulse E18_(3a) appears and the pulse E18_(3a) passes through the gate 18₄ as the erroneous transport control signal E20. This erroneous signal E20 erroneously instructs the transport control logic 10₂, and when transport control logic 10₂ is erroneously signaled, the control logic 10₂, for example, stops the tape transport mechanism 10₁, which again produces the erroneous pulse E18_(3b).

By contrast, when the AND gate 18₄ does not pass the pulse E18₃ while the second signal E18₄ remains logical "0", no erroneous transport control signal E20 appears as illustrated in FIGS. 4 and 5.

FIGS. 7A and 7B are timing charts explaining the operation of the masking circuit 18₅. In these figures, a reference time or a start point of time counting is selected at the rising edge of the instruction signal IS. As shown in FIG. 3, the signal IS is transferred between the tape transport mechanism 10₁ and the transport control logic 10₂. Here the signal IS is to be an instruction for changing the operation mode from playback mode to FF mode. The signal IS causes the logical level change in the transport operation control signal E12 from "1" to "0". FIGS. 7A and 7B may suggest that the masking period, i.e. the pulse width of the second signal E18₄, is at least 200 ms, preferably about 300 to about 400 ms. However, a masking period of 200 ms or even a masking period of 400 ms is not long enough to prevent said erroneous operation completely.

In some tape transport mechanisms, e.g. a mechanism provided with said model PC-X40 the playback signal E10 is muted on a rare occasion after the mode change from playback to FF, for the reason which will be later described with reference to FIG. 8. This muting period is denoted by "TM" in FIG. 7B. From the FIGS. 7A and 7B it may be concluded that the masking period or the pulse width of the second signal E18₄ should be at least 550 ms, and preferably 750 ms, in said model PC-X40.

FIG. 8 explains said muting phenomenon. A mode change lever 20 pivoted by a shaft 22 is seesawed by a rotation plate 24. That is, when the plate 24 rotates to CW, a mechanical interlocking portion 20₁ is moved down by the mechanical driving through rotation of a projection or pin 24₁ inserted in a cam slot 20₂. The portion 20₁ is moved up when the plate 24 rotates to CCW. In the mechanics shown in FIG. 8 optional over-strokes can be obtained with suitable mechanical design.

When the operation mode in the tape transport mechanism 10₁ is changed from playback (point a) to FF or REW (point c), the mechanical motion of the lever 20 starts shifting from points a to c via a point b and stops at the point c. Here L1 denotes a first over-stroke.

On the other hand, when the operation mode is changed from FF or REW to playback, the lever 20 starts shifting from points c to a via a point d and stops at the point a. In this case L2 indicates a second over-stroke.

The over-strokes L1 and L2 help to secure the mechanical operation. Especially the over-stroke L1 causes the head 10_(1a) to detach from the music tape. Such detachment causes said muting phenomenon. With respect to these mechanics is described more detail in Japanese Patent Application No. 25444/79 and Japanese Utility Model Application Nos. 27757/79 and 27758/79.

In the above-mentioned case the first over-stroke L1 is larger than the second over-stroke L2, though the mode change cycle between playback mode to FF (or REW) mode or vice versa is always constant. L1 is greater than L2 (L1>L2) because the time delay for mechanical mode change from FF (or REW) mode to playback mode should necessarily be short, preferably with 30 ms, in order to avoid too long of a masking period.

FIG. 9 shows another embodiment of the invention, or a modification of the configuration shown in FIG. 1 or 3. The components 10 to 18 have already been explained with reference to FIGS. 1 and 3. The explanation will therefore be directed to other components. The signal treatment means 28 provides a first transport control signal E20₁ in accordance with the provision of the timing signal E18 and the erroneous operation preventing signal E22. The signal E20₁ is supplied to a counter 26. The counter 26 produces a count output signal E26 corresponding to the count. The signal E26 is supplied to a coincidence detector 28.

The embodiment of FIG. 9 is also provided with a source number or music number designator 30. The designator 30 provides a designation data E30 to a gating circuit 32. The circuit 32 gates or selects music numbers in accordance with the contents and the logical level of the transport operation indicating signal E12 and thus provides a designation signal E32. The signal E32 is supplied to the detector 28 and is compared with the signal E26 therein. The detector 28 provides a second transport control signal E20₂ when the signal E26 coincides with or corresponds to the signal E32. The signal E20₂ is converted to a pulse signal E36₁ by a pulse shaper 34.

The signal E36₁ is applied to the first input of an OR gate 36₁. The second input of the OR gate 36₁ is supplied with a skip pulse E36₃ derived from a pulse shaper 36₃. The pulse E36₃ corresponds to a skip instruction signal E36₂ obtained from a skip switch 36₂. The OR gate provides an operation mode changing instruction, i.e. the transport control signal E20 to the recording medium transport means 10. The signal E20 is usually employed for a plunger driving pulse to change the mechanical operation mode. Accordingly, the signal E20 is provided with the logical "1" of the signal E36₁ or the signal E36₃, i.e. the signal E20 always occurs when the switch 36₂ is turned-on or when the signal E20₂ is provided. More practically, when the switch 36₂ is turned on, the operation mode of the tape transport mechanism 10₁ (FIG. 3) is changed (PLAY⃡FF or REW).

In a service data of said model PC-X40 the switch 36₂ is indicated by "PLAY/SKIP buttton". The components 36₁, 36₂ and 36₃ constitute a skip instruction means 36. The signal E 20 is level-inverted by an inverter 10₄ and the inverter 10₄ provides the erroneous operation preventing signal E22 to the signal treatment means 18.

The configuration of FIG. 9 can perform an automatic multi-music selection. Suppose the designator 30, which corresponds "Music selection buttons" of said PC-X40, designates music numbers 1, 3 and 5 and that the present operation mode of the tape transport mechanism 10₁ is playback mode. In this case the gating circuit 32 applied with the signal E12 of logical "1" does not pass the designation data E30 of 1, 3 and 5. That is, the designation signal E32 includes the data of 2, 4, 6. . . Then, the second transport control signal E20₂ appears after the music sources of 1st, 3rd and 5th, and the music sources of 2nd, 4th, 6th . . . are skipped by FF opertion. On the other hand, if the mechanism 10₁ is changed from playback mode to FF mode while the 1st music source is played, the signal E12 of logical "0" is applied to the gating circuit 32. In this case the circuit 32 selects the designation data E30 of 1, 3 and 5. That is, the signal E30 includes the data of 1, 3 and 5. Then, the control signal E20₂ appears after the 1st, 2nd and 4th music sources, and the remaining portion of the 1st source and the music sources of 2nd and 4th are skipped, and 3rd and 5th music sources are played back.

FIG. 10 shows a modification of the variable filter means 16 in which the cut-off frequency f_(c) is varied according to the tape transport speed. A supply reel shaft of the tape transport mechanism 10₁ is interlocked with a rotary magnet ring 10_(1b), and the ring 10_(1b) rotates with the supply reel shaft. The ring 10_(1b) therefore rotates during the tape transport.

The rotation of the ring 10_(1b) is detected by a Hall element 10_(1c). An output signal of the element 10_(1c), i.e. the transport speed indication signal E12, is amplified by an amplifier 10_(1d), wave-shaped by a wave shaper 10_(1e) and converted into an analog signal by a D/A converter 10_(1f), and then the filter characteristic changing signal E14 is obtained from the converter 10_(1f). Thus, the signal E14 corresponds to the tape transport speed.

The signal E14 is supplied through a resistor R16₁₄ to a LED 16₄ whose cathode is connected to a negative power source -V_(E). Optically coupled with the LED 16₄ are CdS cells or variable resistance elements R16₁₀ and R16₁₂ which are responsive to the light from the LED 16₄. The element R16₁₂ is connected between the inverted input and output of an amplifier 16₁. The non-inverted input of the amplifier 16₁ is grounded and the inverted input thereof is applied with the reproduction signal E10 through a series circuit of a capacitor C16 and the element R16₁₀. The output of the amplifier 16₁ provides the timing signal E18.

The components 10_(1b) and 10_(1c) constitute a means for issuing a transport speed indicating signal corresponding to the transport speed of the recording medium. The components 10_(1d) to 10_(if) constitute a means for converting the transport speed indicating signal to the filter characteristic changing signal.

The components C16, R16₁₀, R16₁₂ and 16₁ constitute the variable filter means 16. The transfer function G16 of the means 16 can be represented as follows: ##EQU12## where S=j2πf(Hz), T5=C16R16₁₂ and T6=C16R16₁₀. The maximum gain of the function G16 is obtained from the equation (10) as follows: ##EQU13## The cut-off frequency F_(c) at the level of -3dB point from |G16_(max) | is also obtained from the equation (10): ##EQU14##

When the tape transport speed becomes high, the signal level of E14 becomes also high level. This enhances the light intensity from the LED16₄ and reduces the resistance of the element R16₁₀. Accordingly, as evident from equation (12), the cut-off frequency f_(c) continuously rises as the tape transport speed increases. If the optical properties of the elements R16₁₀ and R16₁₂ are substantially the same, the ratio R16₁₂ /R16₁₀ in equation (11) will be almost constant, regardless of the tape transport speed.

FIG. 10A shows a partial modification of FIG. 10. In this modification a rotary magnet ring 10_(1b) is interlocked with a roller 10_(1g) which contact with a magnetic tape. The Hall element 10_(1c) therefore provides a transport speed control signal E12 directly proportional to the tape transport speed. Thus, the variation of the cut-off frequency f_(c) corresponds to real tape transport speed, not to the rotation speed of the reel shaft.

FIG. 11 shows a modification of FIG. 10. In this modification a frequency generator or AC generator 10_(1h) interlocking with a supply reel shaft is used for issuing a transport speed indicating signal E12 corresponding to the transport speed of the tape. A generation output (e.m.f.) of the generator 10_(1h), i.e. the signal E12 is rectified by a rectifier 10_(1i), wave-shaped by a wave shaper 10_(1j) and amplified by an amplifier 10_(1k), whereby the filter characteristic changing signal E14 is obtained from the amplifier 10_(1k). The function of FIG. 11 configuration is substantially the same as that of FIG. 10.

FIGS. 12 to 18 show various modifications of the variable filter means 16. In the case of FIG. 3, the cut-off frequency f_(c) is varied by changing the capacitance "C16₁ ⃡(C16₁ +C16₂)". In the case of FIG. 12, this change is achieved by changing the capacitance "C16₁ ⃡C16₂ " by means of a switch 16₅ which closes in response the signal E14.

In the case of FIG. 13, resistances in the negative feedback loop are changed, instead of changing the capacitors. The output of an amplifier 16₁ is connected to the inverted input thereof via resistor R16₁₉. The inverted input is grounded through a series circuit of a resistor R16_(2a) and a capacitor C16. The junction point of the resistor R16_(2a) and the capacitor C16 is connected to the output of the amplifier 16₁ through a series circuit of resistors R16_(2b) and R16_(1b). Connected between the inverted input of the amplifier 16₁ and the junction point of the resistors R16_(2b) and R16_(1b) is a switch 16₆. When the switch 16₆ is turned on, the composite resistances R16_(2a) //R16_(2b) and R16_(1a) //R16_(1b) are made to correspond to the resistances R16₂ and R16₁ of FIG. 3 or 12, respectively, at this time the cut-off frequency f_(c1) and the maximum gain G16₁ are: ##EQU15##

On the other hand, when the switch 16₆ is turned off and a relation R16_(2a) <<(R16_(2b) +R16_(1b)) is satisfied, the cut-off frequency f_(c2) and the maximum gain G16₂ are: ##EQU16##

In the configuration of FIG. 13, it is possible to arrange G16₁ ≃G16₂. Thus, from equation (14)≃equation (16), the following relation is obtained:

    R16.sub.1b (R16.sub.2a +R16.sub.2b)≃R16.sub.2b (R16.sub.1a +R16.sub.1b)                                              (17)

For example, if the resistors are so selected that R16_(2a) =10K Ω, R16_(2b) =220Ω, R16_(1a) =3.3 MΩ and R16_(1b) =68K Ω, the relation G16₁ ≃G16₂ is practically satisfied.

In the case of FIG. 14 the cut-off frequency f_(c) is varied by changing the resistances "R16_(2a) ⃡R16_(2b) " by means of a switch 16₅.

In the case of FIG. 15 a switch transistor 16₂ is used for the switch 16₅ of FIG. 14. In this case the cut-off frequency f_(c) is varied by changing the resistances "R16_(2a) ⃡(R16_(2a) //R16_(2b))" by the ON-OFF operation of the transistor 16₂.

In the case of FIG. 16 the cut-off frequency f_(c) is varied by changing the resistances "R16_(2a) ⃡R16_(2b) ", and the maximum gain G16 is varied by changing the resistance "R16_(1a) ⃡R16_(1b) ".

In the case of FIG. 17 the cut-off frequency f_(c) is varied by changing the inner resistance of an FET 16₈, and the maximum gain G16 is varied by changing the inner resistance of an FET 16₉. The inner resistances of the FETs 16₈ and 16₉ depend on the potential level of the signal E14₁ as shown in FIG. 19.

FIG. 18 shows a modification of FIG. 17. In this modification the maximum gain G16 is varied by changing the inner resistance of an FET 16₁₀. The gate potential of the FET 16₁₀,i.e. the signal level of a signal E14a, corresponds to -E14 or anti-phased signal of E14.

Although specific constructions have been illustrated and described herein, it is not intended that the invention be limited to the elements and constructions disclosed. One skilled in the art will recognized that other particular elements or subconstructions may be used without departing from the scope and spirit of the invention.

In addition, a more detailed embodiment of this invention is disclosed in the service data of model PC-X40. 

What is claimed is:
 1. A detector for detecting a space between adjacent blocks of data recorded on a recording medium, said detector comprising:(a) recording medium transport means having several operation modes, for transporting a recording medium at different speeds, for providing a reproduction signal corresponding to data recorded on said recording medium and for issuing a transport operation indicating signal identifying the operation mode of said transport means; (b) means for discriminating said transport operation indicating signal and for providing a filter characteristic changing signal in accordance with said transport operation indicating signal; (c) variable filter means, repsonsive to said filter characteristic changing signal, for filtering said reproduction signal and for providing a timing signal, the filtering characteristic of said variable filter means changing in accordance with said filter characteristic changing signal; and (d) signal treatment means, coupled to said discriminating means and responsive to said timing signal, for generating a transport control signal to change the operation mode of the recording medium transport means.
 2. The detector for claim 1 wherein said variable filter means includes a high-pass filter with high-pass frequency characteristics andwherein said variable filter means includes means for changing the low cut-off frequency of said high-pass frequency characteristics of said variable filter means from a first frequency to a second frequency higher than said first frequency when the speed of said recording medium transport means increases.
 3. The detector of claim 1 wherein said reproduction signal includes a no-signal portion, andwherein said variable filter means includes means for providing said timing signal when said no-signal portion is present in said reproduction signal.
 4. The detector of claim 1, wherein said recording medium transport means includes means for changing said transport operation indicating signal in accordance with the speed of said recording medium.
 5. The detector of claim 4 wherein said variable filter means includes a low cut type filter which includes means for changing the cut-off frequency of said low cut type filter in accordance with said filter characteristic changing signal from a predetermined low value to a predetermined high value when the speed of the recording medium transport means increases.
 6. The detector of claim 5 wherein said variable filter means includes a high pass filter of-6 dB per octave and having a predetermined cut-off frequency corresponding to a predetermined speed of said recording medium.
 7. The detector of claim 4 wherein said discrimination means includes means for generating a time constant changing signal,wherein said signal treatment means includes a variable time constant bistable multivibrator, triggered by said timing signal, and means for providing a first signal corresponding to said transport control signal, the time constant of said multivibrator being changed by said time constant changing signal at the same time that said discriminating means provides said filter characteristic changing signal.
 8. The detector of claim 1 wherein said discriminating means also includes means for generating a time constant changing signal and wherein said signal treatment means includes a variable time constant bistable multivibrator triggered by said timing signal and means for providing a first signal corresponding to said transport control signal, said multivibrator including means for changing the time constant of said multivibrator according to said time constant changing signal when said discriminating means provides said filter characteristic changing signal.
 9. The detector of claim 8 wherein said recording medium transport means includes means for transporting said recording medium and control logic for causing said transporting means to change the operation mode of said recording medium transport means, said control logic including means for providing an erroneous operation preventing signal when said control logic changes the recording medium transport means operation mode; andwherein said signal treatment means includes a monostable multivibrator, coupled to receive said erroneous operation preventing signal, for providing a second signal within a predetermined period after said monostable multivibrator receives said erroneous operation preventing signal, and a logical AND circuit coupled to receive said first and second signals for providing said transport control signal corresponding to the logical AND of the first and second signals, whereby the timing of the transport control signal is delayed with respect to the predetermined period of the monostable multivibrator and erroneous operation due to mechanical delay is prevented.
 10. The detector according to claim 1 wherein said recording medium transport means also includes means for transporting said recording medium;wherein said transporting means including means for issuing a transport speed indicating signal corresponding to the transport speed of the recording medium; wherein said discrimination means also includes means for converting said transport speed indicating signal to said filter characteristic changing signal; and wherein said variable filter means also includes a variable impedance element whose impedance varies in accordance with said filter characteristic changing signal.
 11. A detector for detecting a space between adjacent blocks of data recorded on a recording medium, said detector comprising:(a) recording medium transport means, having several operation modes, for transporting a recording medium at different speeds, for providing a reproduction signal corresponding to data recorded on said recording medium, and for using a transport operation indicating signal identifying the operation mode of said transport means; (b) means for discriminating said transport operation indicating signal and for providing a filter characteristic changing signal and a time constant changing signal in accordance with said transport operation indicating signal; (c) variable filter means, responsive to said filter characteristic changing signal, for filtering said reproduction signal and for providing a timing signal, said variable filter means including means for changing the filtering characteristics of said variable filter means in accordance with said filter characteristic changing signal; and (d) signal treatment means, coupled to said discriminating means and responsive to said timing signal, for treating said timing signal, said signal treatment means including a variable time constant bistable multivibrator triggered by said timing signal and producing a first signal, the time constant of said bistable multivibrator being changed by the time constant signal, and said signal treatment means also including means for providing a first transport control signal from said first signal; wherein the operation mode of said recording medium transport means changes in accordance with said first transport control signal.
 12. The detector of claim 11 wherein said variable filter means includes a high-pass filter with high-pass frequency characteristics andwherein said filter characteristic changing means of said variable filter means includes means for changing the low cut-off frequency of the high frequency characteristics of said variable filter means from a first frequency to a second frequency higher than said first frequency when the speed of said recording medium transport means increases.
 13. The detector of claim 11 further comprising skip instruction means for changing the operation mode of said recording medium transport means regardless of said first transport control signal.
 14. The detector according to claim 11 further comprising:means for providing an erroneous operation preventing signal when the recording medium transport means changes its operation mode; and wherein said signal treatment means also includesa monostable multivibrator, triggered by said erroneous operation preventing signal, for providing a second signal within a predetermined period after said monostable multivibrator is triggered, and logical AND circuitry coupled to said first and second signals for providing said first transport control signal; whereby said first transport control signal is delayed with respect to the predetermined period of the monostable multivibrator and erroneous operation due to mechanical delay is prevented.
 15. The detector according to claim 11, wherein said recording medium transport means includes means for transporting said recording medium and means for issuing a transport speed indicating signal corresponding to the transport speed of the recording medium;wherein said discrimination means includes means for converting the transport speed indicating signal to said filter characteristic changing signal; and wherein said variable filter means includes a variable impedance element whose impedance varies in accordance with said filter characteristic changing signal.
 16. The detector of claim 15 wherein a cut-off frequency ratio N of the variable filter means is defined as ##EQU17## where V2 denotes a constant tape transport speed, V1 a speed of high speed tape transport, f_(c2) a cut-off frequency of the filter means when the tape transport speed is V2, and f_(c1) a cut-off frequency of the filter means when the tape transport speed is V1; andwherein said discrimination means includes means for altering f_(c1) and f_(c2) in response to said transport operation indication signal.
 17. The detector according to claim 11, wherein a cut-off frequency ratio N of the variable filter means is defined as ##EQU18## where V2 denotes a constant tape transport speed, V1 a speed of high speed tape transport, f_(c2) a cut-off frequency of the filter means when the tape transport speed is V2, and f_(c1) a cut-off frequency of the filter means when the tape transport speed is V1; andwherein said discrimination means includes means to alter f_(c1) and f_(c2) in response to said transport operation indication signal. 